This invention relates to a projection type image display device utilizing an oil film including a rotating concave mirror, to the surface of which an oil film is applied, and a schlieren optical system, and in particular to an image display device including an elimination circuit, which prevents spurious images from being displayed.
Heretofore, in a projection type image display device utilizing an oil film, the cause of the spurious image signal generation at a variation point of the brightness of the image signal in the vertical direction has not been elucidated yet and no image corrections for eliminating the spurious image signal have been effected.
Further a representative example of the projection type image display device utilizing an oil film is called eyedhole and is known as one of the light bulb projection devices. (cf. Television Image Technique Handbook p. 194 and p. 1054, published by Ohm Publishing Co., 1980)
Here the principal cause of the spurious image signal generation at a variation point of the brightness of the image signal in the vertical direction in a projection type image display device utilizing an oil film will be described by referring to waveforms indicated schematically in FIGS. 1A to FIG. 1C.
FIG. 1A shows the level in the vertical direction of the input image signal V.sub.A with respect to the scanning line and FIG. 1B shows a recessed waveform V.sub.B of the surface of the oil film, depending on the vertical level indicated in FIG. 1A.
The light intensity is controlled, responding to the inclination angle of the oil film formed by this recessed waveform V.sub.B. At this time, on the basis of the relation between the restoring force of the surface of the oil film when it is deformed and the interval to the succeeding scanning line the inclination angle of the oil film at the position of the n-th scanning line, which is the variation point of the brightness in the vertical direction (point where the brightness varies from its low level to its high level), becomes greater than its suitable value. For this reason a spurious image signal V.sub.C is produced, as indicated in FIG. 1(C).
The generation of this spurious image signal occurs more remarkably in particular with decreasing interval between scanning lines adjacent to each other on the surface of the oil film. Since the image scanning area on the oil film should be reduced, in other to display hyperfine images (hivision), for which the number of scanning lines per 1 frame is increased, which has been developed, or to reduce the size of a projection type image display device, which is the object of this invention, the interval between two scanning lines adjacent to each other becomes small. For this reason the generation of the spurious image signal increases. Therefore it becomes necessary and inevitable to eliminate this spurious image signal.